1. Field of the Invention
This invention relates to an optical disc formed by bonding a pair of optical disc substrates and also to a disc cartridge containing such an optical disc.
This application claims priority of Japanese Patent Application No. 2004-066237, filed on Mar. 9, 2004, the entirety of which is incorporated by reference herein.
2. Description of Related Art
Optical discs have been and being popularly used as information recording mediums for information processing apparatus such as personal computers, recording/reproduction apparatus such as optical disc players and video cameras and television game machines.
Efforts have been and are being paid to improve the recording density and improve the recording capacity of optical discs of the type under consideration. For example, optical discs with a diameter of 8 cm having a recording capacity of 1.4 gigabytes (GB) and capable of recording a movie of about 30 minutes have been offered. Furthermore, in the case of DVDs that are a type of optical disc, DVDs with a diameter of 8 cm having a recording capacity of 1.8 GB and capable of recording a movie of about 2 hours have been proposed.
On the other hand, portable type recording/reproduction apparatus such as optical disc players and video cameras and also portable type television game machines adapted to use optical discs as recording mediums are commercially available. Such portable appliances are required to be more downsized and thin in view of convenience of mobility and easy use.
Thus, there is a strong demand for downsized optical discs having a large capacity and adapted to be used as recording mediums for recording/reproduction apparatus and television game machines that are also downsized and made thin.
To meet this demand, optical discs having a plurality of recording layers or formed by bonding a plurality of disc substrates, each having a recording layer, have been proposed in an attempt to realize a large capacity and compactness.
Japanese Patent Application Laid-Open Publication No. 9-282712 (Patent Document 1) describes an optical disc having a plurality of recording layers of the type under consideration. The optical disc described in Patent Document 1 is formed by bonding a pair of disc substrates, each having a signal recording layer at a side thereof, by way of an intermediate layer that is an adhesive layer.
While the optical disc described in Patent Document 1 can provide an increased recording capacity because it is formed by bonding a pair of substantially flat disc substrates, it cannot satisfactorily contribute to downsizing the appliance that uses the optical disc as recording medium. In other words, a clamp mechanism or the like that holds an optical disc as described in Patent Document 1 at a surface thereof while the optical disc is mounted on a turn table at the opposite surface is required in order to support the optical disc on the disc rotary drive mechanism of a recording/reproduction apparatus so as to allow it to rotate integrally with the disc rotary drive mechanism. Then, it is difficult to reduce the thickness of the appliance.